Folding is an essential step of biosynthesis of proteins because biological function can only be obtained by specific structures. This proposal focuses on the question how a recently discovered enzyme activity termed peptidyl-prolyl cis-transisomerase influences the triple helix formation of collagens in vitro and in vivo. The activity is to be purified to homogeneity from pig kidneys and characterized in regard to molecular weight, hydrodynamic parameters, aminoacid composition, aminoterminal aminoacid sequence, spectroscopic properties and enzyme kinetics. Enzymatic activity is determined against the cis to trans isomerization of succinyl-Ala-Ala-Pro-Phe-4- methlycumaryl-7-amide by means of a two-step process using chymotrypsin as the trans substrate cleaving activity. This enzyme probably helps folding of protein which has been shown to be rate limited by isomerization of proline containing peptide bonds. To test this hypothesis the influence of peptidyl-prolyl cis- trans isomerase on in vitro folding of type III collagen is investigated. Refolding of type III collagen was shown to be rate limited by cis to trans isomerization of peptide bonds. The influence of ionic strength and pH on the kinetics of peptidyl- prolyl cis-trans isomerase is investigated as well as the intercellular localization by immunoelectronmicroscopy. To see whether the enzyme is actively involved in in vivo folding during biosynthesis, the enzyme level is investigated for correlation with 4-prolyl hydroxylase in a developing system with varying degrees of collagen synthesis. What happens if folding is impaired has been shown recently in cases of Osteogenesis Imperfecta where a single aminoacid substitution for a glycine residue in the pro alpha 1 chain of type I procollagen leads to procollagen molecules with decreased melting temperature, increased posttranslational, modification altered rate of secretion, increased intracellular degradation, and decreased collagen production.